2002 Michelson Interferometry Summer School

Integrated Optics for astronomical Interferometry

Pierre Kern (Laboratoire d'Astrophysique de Grenoble)

Abstract:

Integrated Optics technics is able to provide astronomical instrumentation with very powerful tools. In the case of single mode interferometry many critical functions can be realized using coin size optical components while optical wave-guides can be implemented on silicon or glass substrates. Initially developed for telecommunication and micro-sensors applications, many integrated optics components are routinely produced by the industry. Fruitful developments were initiated for astronomy using available components to obtain interference fringes.

In this presentation we will first revue basic principles and realization procedure. Some typical available functions will be presented. Available designs allow an easy implementation of beam combiners and associated photometric calibration on the same component. Validations using of off-the-shelves components have been done for typical telecommunication wavelengths, mainly between 1.3 or 1.5 micrometer. Fringe contrasts up to 95% can be obtained in laboratory conditions within the atmospheric J, H and K band. Results obtained in laboratory and on the sky using 2 and 3 telescope beam combiner will be presented.

We present also current developments for specific astronomical application, especially for 3 telescope and more beam combiners. It can be pointed out that the specific constraints for the integrated optics designer are to optimize overall functional optical throughput, and the broad band capacity, as main telecommunication applications use laser sources. In order to be fully applicable to astronomical applications some further developments are required as the extension of the scope of planar optics to higher wavelengths and the design of new specific integrated optics functions ( dichroic separation, optical path modulation, active polarization modification, ...)

We show how integrated optics can be proposed for critical experiment:

beam combination of large network of telescope as CHARA, the Keck interferometric mode or the VLTI
nulling interferometry where all specifications are very stringent (polarization behavior, optical path equalization, modal filtering).

In this last case direct light injection in the substrate can be mandatory to avoid fiber optics constraint effects. Concepts of fully integrated instrument based upon integrated optics can be imagined. A unique optical chip includes a complex multi-way beam combiner and the required associated functions. This compact component, which benefits from low sensitivity to external constraints and from high stability, can be directly implemented in a cryostat in front of a detector. In this case, the interface with the incoming beams is limited by injection optical fibers as cryostat feed through. Such a concept reduces the instrumental alignment constraints to wave-guide injections. Some integrated optics concepts are also very attractive for space applications to be compatible with some specific spacecraft constraints (dimension, load, stability).

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Integrated Optics for astronomical Interferometry

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